UPDATE 1-Former WorldPay boss keen to buy RBS WorldPay

* Former WorldPay founder says wants to buy RBS WorldPay

* RBS WorldPay could be worth about 1.5 billion pounds

* RBS forced to sell business by European regulators

* Lynk Systems founder also interested in U.S. part (Adds detail, background)

By Steve Slater and Paritosh Bansal

LONDON/NEW YORK, Nov 6 (Reuters) - A founder of part of payment-processing business RBS WorldPay is interested in buying all of the business back from Royal Bank of Scotland (RBS) (RBS.L) after European competition authorities told the UK bank to sell it.

RBS WorldPay is one of the leading payment processing businesses globally and could have a price tag of about 1.5 billion pounds ($2.5 billion).

Nick Ogden, chairman and chief executive officer of Voice Commerce Group, said he had backing from up to three private equity firms to finance a deal.

"Yesterday, we formally approached RBS to buy the business back from them," Ogden told Reuters. The interest had been passed on to UBS (UBSN.VX), who are handling the sale, he said.